The manufacture of semiconductors typically requires precise control of fluids (e.g., gasses, liquids, gas-vapor mixes) throughout the manufacturing process. Measurement and regulation of fluid flow is typically achieved by the use of a thermal mass flow controller (MFC). The MFC has a flow meter or sensor, which generally operates by directing fluid flow through a capillary tube path that runs parallel to a bypass area of the main flow path. Two thermally sensitive resistors are wound around the capillary tube. As fluid travels through the capillary tube, heat is imparted to the fluid and conducted away from the resistors, causing the resistance of each of the resistors to change and the temperature of the fluid to change. Based on the difference in resistance between the two resistors, a controller executing a control algorithm can determine the flow through the MFC. The flow sensor is coupled to a control valve and feedback circuit so that the flow rate can be electronically set or manipulated. Solenoid activated valves are often used as control valves because of their simplicity, quick response, robustness and low cost.
As illustrated in FIGS. 1A and 1B, thermal mass flow controller 100 includes a block 110, which is the platform on which the MFC's components are mounted. The block has a fluid inlet 120 and a fluid outlet 130 connected by the channels 122 that form the main flow path of the fluid. Thermal mass flow meter 140 and valve assembly 150 are mounted on the block. As shown in FIG. 1B, all of these components along with the associated control electronics 160 (typically formed on a printed circuit board) are contained within an outer cover 162, commonly referred to as a “can.”
In the illustrated example, the thermal mass flow meter 140 includes a pressure dropping bypass 142 through which a majority of fluid flows, and a thermal flow sensor 146 through which a smaller portion of the fluid flows. The thermal flow sensor 146 is contained within a sensor housing 102 mounted on a mounting plate or base 108. Sensor 146 is a small diameter tube, referred to as a capillary tube, with a sensor inlet portion 146A, a sensor outlet portion 146B, and a sensor measuring portion 146C about which two resistive coils or windings 147, 148 are disposed.
In operation, electrical current is provided to the two resistive windings 147, 148, which are in thermal contact with the sensor measuring portion 146C. The heat generated by the resistive windings 147, 148 is used to heat the fluid flowing therein to a temperature that is above the temperature of the fluid flowing through the bypass 142. As known to those skilled in the art, the rate of flow of fluid in the flow sensor 146, which is proportional to the rate of flow of fluid through the mass flow controller 100, may be determined in a number of different ways, such as, by a difference in the resistance of the resistive windings, by a difference in the amount of energy provided to each resistive winding to maintain each winding at a particular temperature or at a particular temperature above ambient temperature, etc. Examples of the ways in which the flow rate of a fluid in a thermal mass flow meter may be determined are described, for example, in commonly owned U.S. Pat. No. 6,845,659 B2, which is hereby incorporated by reference.
A fundamental problem with typical MFCs is that, due to the operating principal of the flow sensor, external temperature differentials can greatly impact the accuracy of the flow measurement. For example, the solenoid valve and electronics generate heat during operation. Since the components of the MFC are all thermally connected, the heat generated by the valve 150 and electronics can cause a temperature gradient across the MFC, which in turn causes a temperature gradient across the thermal mass flow meter 140, which in turn causes a temperature gradient along the flow sensor 146. Because the flow sensor actually operates by responding to a heat/resistance difference between the two coils, a temperature gradient across the flow sensor will be interpreted as fluid flow and will result in variations and errors in flow measurements.
The effects of externally created thermal gradients (for example, those caused by the normal operations of other MFC components) on the sensor readings can be fairly large. For example, the sensor output on a prior art MFC can vary by almost 1% of full scale when the valve is changed from “off” to “purge.” The effect of a temperature change can also take two hours to dissipate because of the large thermal masses involved.
One way of compensating for the temperature gradient effect is to build an algorithm into MFC firmware that continually calculates the amount of power going into the valve and uses that value to estimate and compensate for the temperature gradient. This approach can be used to reduce output variation down to about 0.1% of full scale. However, for modern semiconductor manufacturing methods it is desirable to reduce output variation by another order of magnitude.
One prior art method of mechanically reducing the sensor temperature gradient is seen in U.S. Pat. No. 6,779,394 to Ambrosina et al. for “Apparatus and Method for Thermal Management of a Mass Flow Controller.” Ambrosina teaches a sensor housing which makes contact with the base plate at only one central point. By having only one centrally located thermal pathway, the heat conduction is somewhat equalized for both sides of the housing and the effects of the thermal gradient are minimized.
However, the solitary central connection point in Ambrosina tends to allow the sensor housing to flex (for example, by rocking in one direction or another). This flex can cause undesirable stress on the capillary tube. Also, in other common types of MFCs, components such as pressure transducers (to measure the pressure upstream from the bypass) are commonly located under the sensor base. As the base is tightened down onto the block, this tends to cause the base to flex. If the sensor housing is only attached to the base at one center point, flexure of the base can cause an unacceptable stress on the two ends of the capillary tube. Further, the Ambrosina design may reduce the actual gradient on the housing (and thus the sensor) but it does not sufficiently reduce the total heat conduction from the base. Even in the absence of a gradient, it is desirable to minimize the total amount of heat conducted to the sensor.
What is needed is an improved mass flow meter designed to minimize the effects of a thermal gradient along the sensor base and to minimize heat conduction from the sensor base to the sensor elements.